A device powered by rechargeable batteries may include several battery cells to achieve the voltage and/or current levels used by the device. For example, if a rechargeable battery cell has a nominal output voltage of 1 Volt, the a device having a 2 Volt operational level may include two battery cells placed in series. In another example, if a rechargeable battery cell has a nominal output current of 100 milliamps, then a device having a 400 milliamp operational level may include four battery cells in parallel. Battery cells in parallel and series may be combined to reach the operational levels of the device.
The battery cells may be grouped with circuitry for balancing the charge levels in the battery cells to form a battery pack system module. Multiple battery pack system modules may be combined in series or parallel to further increase the output voltage and output current available to a device coupled to the battery pack system modules. Although battery cells within a battery pack system module may be balanced by using balancing circuitry within the battery pack system module (referred to as intra-module balancing), there is a need for balancing battery pack system modules to other battery pack system modules (referred to as inter-module balancing).
One conventional solution for providing inter-module balancing includes shorting out a battery pack system module within a battery pack system with a bypass switch. FIG. 1 is a block diagram illustrating a conventional battery pack system module with a bypass switch. A battery pack system 100 includes battery pack system modules 110, 130. The module 110 includes a first group of battery cells 114 having a battery cell 116 coupled in parallel with a battery cell 118. The module 110 also includes a second group of battery cells 124 having a battery cell 126 coupled in parallel with a battery cell 128. The first group 114 is coupled in series with the second group 124.
When a bypass switch 112 activates, current through the module 110 is diverted away from the battery cells 116, 118, 126, and 128. To prevent short circuiting of the battery cells 116, 118, 126, and 128, a resistor 120 is coupled in series with the switch 112. However, the resistor 120 consumes power and generates heat in the system 100 through Joule heating. The heat generated by the resistor 120 may result in dangerous conditions within the system 100. For example, the heat may lead to a fire involving the battery cells 116, 118, 126, and 128.
Heat generated by the resistor 120 may be problematic where the system 100 is operating in an isolated environment. For example, on an undersea vehicle such as a submarine, battery pack systems may be isolated in a pressurized compartment. Thus, heat dissipated by the resistor 120 may not be carried away and result in dangerous conditions for the vehicle and operator of the vehicle.
Additionally, when one of the modules 110 or 130 of the system 100 becomes defective, the defective module may be replaced with a new module. The new module may be at a significantly different charge than existing modules of the system 100. In a conventional system, balancing of the replacement module with the existing modules may occur over a long period taking days or weeks to reach balance. During this time the system 100 may be unavailable for use. In the above example if one module in the vehicle is replaced, the vehicle may not be ready for operation until the modules are fully-charged and balanced. If the balancing operation consumes days or weeks, the vehicle may be out of service for this entire time period.